After these studies were completed he became an assistant of Professor Julius Hirschberg, the eminent ophthalmologist, physician and historian. He worked for two years in Hirschberg's private eye clinic, and in 1889 returned to America as a well-trained ophthalmologist. He became Associate Professor of Ophthalmology at the College of Physicians and Surgeons in Baltimore. He also opened a practice in Baltimore's Eye, Ear and Throat Hospital.
Dr. Harry Friedenwald's Zionist activities were wide ranging. He took part in the Jewish Congress' meetings. He also visited Palestine, where he helped to treat patients and advised on local medicine. His son Jonas was one of the founding fathers of the medical school at The Hebrew University of Jerusalem.
Dr. Friedenwald's interest in the history of medicine was inspired after his father. Dr. Aaron Friedenwald gave a lecture in 1896 on: "The contribution of the Jews to the science of medicine".
Dr. Harry became a historian of Jewish medicine and published one of the most important books ever written on the subject: "The Jews and Medicine" (1944). He also became an enthusiastic book collector, and his collection is considered one of the largest collections dealing with the history of Jewish medicine. The collection was donated to the Jewish National and University Library in Jerusalem. Dr. Harry Friedenwald died on April 9, 1950.